You haven't made them lighter, you've made them more bouyant. A ton of feathers weighs the same as a ton of rocks, but a ton of rocks is far more aerodynamic.
And for buoyancy to matter, the trucks would need to be huge (zepelin style).
You still need the same engine output to accelerate 100 tons, but you only have the traction you would get from an 80 ton load. That's a bad thing. Meanwhile, you save a miniscule amount of energy by having less-loaded tires.
Anyway, you need to compress hydrogen to store any meaningful amount, which increases the density and makes it like a normal gas/liquid instead of making you more buoyant. So it's not even possible.
But actually, braking distance vs mass is more like a bathtub curve. With very little weight (talking toy cars here) it's hard to grip a surface, meanwhile very large masses like trucks have huge amounts of kinetic energy to disperse through their contact patches, leading rubber to melt, which reduces its stickiness beyond a certain point. Trucks end up with longer braking distances in practice.
You have the same weight, but less traction. That is not the same as reducing weight, in which case the two cancel out as you said.